The present invention relates to electrolyte solution compositions and lithium-ion batteries employing these electrolyte solutions. These electrolytes feature lower volatility than solutions known in the art while retaining excellent battery performance using graphite based negative electrode active materials
Lithium-ion batteries are now under intensive development around the world to provide a new generation of secondary, or rechargeable, batteries. Whatever the specific design approach, all have in common an electrolyte comprising an ionic species and an aprotic liquid, referred to herein as an electrolyte solvent, to provide a physical medium through which the ionic species can move. Commercial lithium-ion batteries generally exhibit a high open-circuit voltage, typically 3.6 to 3.8 volts. This means that during charging, a voltage as high as ca. 4.2 volts will normally be died, with localized transient voltages even higher. Secondary lithium-ion batteries are distinguishable over the primary lithium metal batteries of the art not only in that the voltages to which battery components are exposed are generally higher, but also in that the battery components of a lithium-ion battery must endure repeated exposure to these highly oxidizing conditions during numerous charge/discharge cycles.
Every component of the lithium-ion battery must be able to endure the repeated exposure to the very high electrochemical oxidation and reduction potentials which these voltages represent. Many well-known electrolyte solvents suitable for use in other types of batteries simply do not exhibit the requisite stability for lithium-ion battery use. There appears to be no generalized scheme accepted in the art beyond trial and error for selecting those electrolyte solvents which will exhibit the requisite stability. In practice, this has constrained the choice of electrolyte solvents employed in the art of lithium-ion batteries to the acyclic and cyclic organic carbonates, primarily dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and ethylene carbonate (EC), and monoesters such as methyl acetate (MA), ethyl acetate (EA), methyl formate (MF), methyl propionate (MP), ethyl propionate), and gamma-butyrolactone (GBL) as described in B. A. Johnson, and R. E. White, xe2x80x9cCharacterization of Commercially Available Li-ion Batteriesxe2x80x9d, Journal of Power Sources, 70, 48-54, (1998). Most often, these electrolyte solvents are used in combinations comprising a cyclic organic carbonate, usually EC or PC, and an acyclic carbonate, usually DMC, DEC, or EMC, as disclosed in U.S. Pat. No. 5,525,443 to Matsushita. These combinations have been found in practice to achieve an excellent combination of desirable properties such as high ionic conductivity over a wide temperature range and relatively low volatility while achieving excellent lifetime and performance in lithium-ion batteries. The state-of-the-art is also well described in xe2x80x9cOrganic Electrolytes for Rechargeable Lithium Batteries,xe2x80x9d by M. Morita, M. Ishikawa, and Y. Matsuda, in Ch. 7 of Lithium-Ion Batteries, Fundamentals and Performance, Ed. By M. Wakihara and O. Yamamoto, Wiley VCH, 1998.
The patent art disclosing electrolyte solvents for use in lithium-ion batteries is voluminous. The disclosed electrolyte solvents suitable for use in lithium-ion batteries fall into three broad categories: (1) halogen-substituted organic carbonates such as 2-fluoroethylene carbonate, (2) mixes of organic carbonates with acyclic or cyclic esters such as EC+DMC+methyl formate, and (3) unsaturated organic carbonates such as vinylene carbonate.
Representative of the scope of the art are the following: U.S. Pat. No. 5,192,629 wherein is disclosed mixtures of ethylene carbonate and dimethyl carbonate in ratios of from 20/80 to 80/20; U.S. Pat. No. 5,474,862 wherein is disclosed a combination of cyclic and acyclic organic carbonates with CH3CHC(O)OR where R=C1 to C3 alkyl; U.S. Pat. No. 5,571,635 wherein is disclosed a combination of EC, PC, and chloroethylene carbonate; U.S. Pat. No. 5,578,395, wherein is disclosed a combination of EC, dimethoxyethane (DME), and butylene carbonate (BC); U.S. Pat. No. 5,626,981, wherein is disclosed a combination of a cyclic and acyclic organic carbonate, and an unsaturated organic carbonate such as vinylene carbonate (VC); U.S. Pat. No. 5,626,985, wherein is disclosed a combination of a cyclic and an acyclic organic carbonate with 40-80% ether such as DME; U.S. Pat. No. 5,633,099 wherein is disclosed acyclic asymmetric fluorine-substituted organic carbonates; U.S. Pat. No. 5,659,062, wherein is disclosed CH3OC(O)OCH2CR3 where RC=C1 to C2 alkyl, F-substituted alkyl, or F; and, U.S. Pat. No. 5,773,165, wherein is disclosed EC/PC (50-60%) in combination with GBL (10-25%), DMC, and EC/MA.
In every case in the art, an acyclic ester or acyclic organic carbonate is a required component in the composition in order to achieve the ionic conductivity thought to be required for most lithium-ion battery applications. However, the acyclic esters and acyclic organic carbonates are undesirably fugitive and flammable under some conditions contemplated for battery manufacturing. There is a clear need in the art for high conductivity electrolyte compositions having reduced volatility and flammability.
Webber, U.S. Pat. No. 5,219,683, discloses the use of solvents of the type Yxe2x80x94Oxe2x80x94Xxe2x80x94Oxe2x80x94C(O)xe2x80x94R where R is a C1-C10 alkyl group, X is a C1-C8 acyclic group and Y is a C1-C10 alkyl group or a carbonyl group. Their preferred composition includes ethylene glycol diacetate preferably mixed with propylene carbonate and a salt such as lithium trifluoromethane sulfonate. Claimed is the use of diacetate solvents in lithium primary batteries such as the Li/FeS2 battery. The maximum voltage to which the solvents are exposed is about 2 volts.
Horiba et al., JP 86017106, employs diesters from dicarboxylic acids in lithium primary batteries. The battery exemplified had an open circuit voltage of 2.9 V, and was not subject to recharging.
Liu et al., WO 99/44246, describes lithium-ion polymer batteries prepared using plasticizers based on dialkyl adipate dibasic esters. According to Liu et al., the adipate ester plasticizer is substantially removed from the battery by an extraction process prior to addition of battery electrolyte. However, Liu et al. teaches that residual adipate ester plasticizer up to as much as 20 wt-% does not affect battery performance.
Chang in WO 00/01027 discloses the use of malonate diesters containing no alpha hydrogens as electrolyte solvent in lithium-ion batteries.
The present invention provides for an electrode composition comprising a lithium electrolyte solution in ionically conductive contact with a graphite-based electrode-active material, wherein the solution comprises a lithium electrolyte and a solvent represented by the formula
R1C(O)OR2OC(O)R3xe2x80x83xe2x80x83(I)
or by the formula
R1OC(O)R2C(O)OR3xe2x80x83xe2x80x83(II)
where R1 and R3 each independently designates an acyclic alkyl radical of 1-4 carbons, C(O) designates a carbonyl radical, and R2 is an alkenyl radical of 2 or 3 carbons.
The present invention further provides for a lithium-ion battery comprising a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and an electrolyte solution comprising a solvent, and lithium ions, at least one of said anode, cathode, or separator being in ionically conductive contact with said electrolyte solution; and said solvent being represented by the formula
R1C(O)OR2OC(O)R3xe2x80x83xe2x80x83(I)
or by the formula
xe2x80x83R1OC(O)R2C(O)OR3xe2x80x83xe2x80x83(II)
where R1 and R3 each independently designates an acyclic alkyl radical of 1-4 carbons, C(O) designates a carbonyl radical, and R2 is an alkenyl radical of 2 or 3 carbons.
For the purposes of the present invention, the term xe2x80x9celectrolyte solventxe2x80x9d will refer to any composition of matter which is liquid under the conditions of use in a lithium battery and which serves to provide the medium in which one or more ionic species is dissolved and through which ionic species are transported while the battery is undergoing electrical charge or discharge. The term xe2x80x9clithium electrolytexe2x80x9d will refer to any composition of matter which provides lithium-ions for dissolution in and transport through the electrolyte solvent. The term xe2x80x9celectrolyte solutionxe2x80x9d will refer to the electrolyte solvent having dissolved in it lithium-ions as provided by the lithium electrolyte.
It is found surprisingly in the present invention that certain esters having two or more ester groups formerly known in the art only as suitable solvents for primary lithium batteries, are highly suitable for the considerably more demanding oxidative environment of rechargeable lithium-ion electrochemical cells. Esters having two or more ester groups, characterized by desirably higher boiling points than the monoesters and acyclic organic carbonates formerly employed in lithium-ion batteries, are now found to be preferred replacements therefor, preferably in combination with cyclic organic carbonates, to meet the need for electrolyte solvents with reduced flammability and volatility while continuing to impart high ionic conductivity and high oxidative stability in secondary lithium-ion batteries. The esters of the present invention are employed to replace the monoesters and acyclic organic carbonates of the art, in whole or in part, in the ionically conductive components employed in lithium-ion batteries.
Esters suitable for the practice of the present invention are represented by the formula
R1C(O)OR2OC(O)R3xe2x80x83xe2x80x83(I)
or by the formula
R1OC(O)R2C(O)OR3xe2x80x83xe2x80x83(II)
where R1 and R3 each independently designates an acyclic alkyl radical of 1-4 carbons, C(O) designates a carbonyl radical, and R2 designates an alkenyl radical of two or three carbon atoms. Preferably, R1 and R3 are the same; more preferably R1 and R3 are methyl or ethyl groups, and n=2. Most preferably, the diester is dimethyl succinate, CH3)C(O)CH2CH2C(O)OCH3. 
In one embodiment of the invention, an electrolyte solvent is formed by combining at least one diester suitable for the practice of the invention with a cyclic carbonate, preferably propylene carbonate or ethylene carbonate, in a volume ratio of 90:10 to 30:70. In the preferred embodiment, ethylene carbonate and dimethyl succinate are combined in a volume ratio of 67:33 respectively.
In another embodiment, at least one diester suitable for the practice of the invention is combined with at least one component of a lithium-ion battery, the components being a positive electrode, a negative electrode, and a separator in accord with the teachings of the art as practiced with other liquid electrolyte solvents. In the case of the positive and negative electrodes, the electrolyte solvent is mixed with the electrode-active material and any adjuvants thereto according to the practice in the art. In the case of the separator, if the separator is a porous body, the electrolyte solvent is imbibed within the pores. In the case of a semipermeable membrane, the electrolyte solvent is absorbed by the membrane. In the case of an ionomeric membrane, the electrolyte solvent is absorbed by the ionomer.
The electrolyte solvent of the invention must be in ionically conductive contact at least with the positive electrode, the negative electrode, or the separator in order for the electrochemical processes to take place. Normally, the electrolyte solvent will be in ionically conductive contact with all three.
In the practice of the invention, the electrolyte solvent must be combined with one or more electrolytes which will provide ions to the electrolyte thus rendering it ionically conductive. Suitable electrolytes include low molecular weight lithium salts and ionic polymers, known as ionomers. Suitable low molecular weight lithium salts include both organic and inorganic salts such as LiPF6, LiBF4, LiClO4, LiAsF6, LiN(SO2CF3)2, LiN(SO2CF2CF3)2, LiC(SO2CF3)3, among others. The molar concentration of the lithium-ions in the electrolyte solution may be from 0.1 to 3.0 M, with a preferred range of 0.5 to 1.5 M.
When the ionic species is an ionomer, it may still be desirable to add an amount of low molecular weight lithium salt to the electrolyte solvent in concentrations ranging from 0.01 to 1.0 M.
The lithium battery of the present invention can be a liquid-cell which uses a porous polyolefin separator sandwiched been the electrode film layers such as are described in xe2x80x9cPerformance of the First Lithium-ion Battery and Its Process Technology,xe2x80x9d by Y. Nishi, Ch. 8 of Lithium-Ion Batteries, Fundamentals and Performance, Ed. By N Wakihra and O. Yamamoto, Wiley VCH, 1998. In one embodiment, the lithium battery of the present invention is a cell which uses a polymer electrolyte both as the separator layer and within the electrode film layers thus allowing lamination and assembly of thin-film prismatic batteries. In one embodiment, the polymer electrolyte may comprise a non-ionic polymer, such as described in U.S. Pat. No. 5,456,000, and the electrolyte solvent of the invention. In a further embodiment, the polymer electrolyte may comprise an ionic polymer, such as the perfluorinated sulfonate ionomer described in Doyle et al., WO 98/20773, and the electrolyte solvent of the invention.
In the electrode composition of the invention, a negative electrode is formed by combining at least one ester suitable for the practice of the invention with a graphite-based electrode-active material, and a lithium electrolyte. By xe2x80x9cgraphite-basedxe2x80x9d is meant an electrode-active material which is substantially made of graphite but which may contain such interstitial dopants and other additives and substituents such as are known in the art. Numerous methods for combining the elements of the composition are known in the art, and any convenient method can be used. These methods include tumble blending, melt blending, or sequential film fabrication and soaking in or injection of the electrolyte solution.
Preferred graphite-based electrode-active materials are mesocarbon microbeads such as MCMB available from Osaka Gas or carbon fibers such as Melblon(copyright) available from Petoca which are capable of achieving  greater than 280 mAh/g reversible capacity for lithium insertion. Other suitable carbon-based electrode active materials include graphite flakes, PCG graphite available from Osaka Gas, petroleum coke, hard carbon, and natural graphite. In one embodiment, the lithium electrolyte may be either a lithium salt, preferably LiPF6, LiBF4, LiClO4, LiAsF6, LIN(SO2CF3)2, LiN(SO2CF2CF3)2, LiC(SO2CF3)3, most preferably, LiPF6.
In an alternative embodiment, the lithium electrolyte is an ionomer. The preferred ionomer is a polymer comprising monomer units of vinylidene fluoride (VF2) further comprising 2-50 mol-% of monomer units having pendant groups comprising the radical represented by the formula
xe2x80x94(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Xxe2x88x92(L+)(Y)c(Z)d
wherein R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted by one or more ether oxygens; a=0, 1 or 2; b=0 to 6; X is O, C, or N with the proviso that c=d=0 when X is O, c=d=1 when X is C, and c=1 and d=0 when X is N; with the further proviso that when X is C, Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)Rf, C(O)R3, and cycloalkenyl groups formed therewith wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally substituted with one or more ether oxygens; R3 is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted; Y and Z are the same or different; or, when d=0, Y may be an electron-withdrawing group represented by the formulaxe2x80x94SO2Rfxe2x80x2 where Rf is the radical represented by the formula xe2x80x94(Rfxe2x80x3SO2Nxe2x88x92((Li+)SO2)mRfxe2x80x2xe2x80x3 where m=0 or 1, and Rfxe2x80x3 is xe2x80x94CnF2nxe2x80x94 and Rfxe2x80x2xe2x80x3 is xe2x80x94CnF2n+1 where n=1-10, optionally substituted with one or more ether oxygens.
Preferably, R is trifluoromethyl, Rxe2x80x2 is F, a=1, b=1, when X is C, Y and Z are CN or CO2R3 where R3 is C2H5, while when X is N, Y is preferably SO2Rf where Rf is CF3 or C2F5.
The preferred ionomer of the invention may be synthesized according to the methods taught in copending U.S. Pat. No. 6,025,092 and WO 99/45048 which are herein incorporated by reference to their respective entirety.
In a preferred embodiment, the electrode composition will additionally contain a polymeric binder and an electronically conductive additive such as carbon black such as Super P carbon black (MMM Carbon). In a preferred embodiment wherein the separator is a PVDF/HFP copolymer membrane, the preferred binder is PVDF/HFP. In an alternative preferred embodiment wherein the separator is a preferred ionomer of the invention, the preferred binder being the same or a closely related ionomer.
A preferred electrode of the invention, which is a negative electrode suitable for use in the lithium-ion cell of the invention, is formed by combining a diester with a graphite-based electrode-active material, carbon black, and the preferred ionomer of the invention in proportions of 62 parts graphite, 4 parts carbon black, 10 parts ionomer, and the remainder a preferred electrolyte solvent of the invention to form the preferred electrode composition. The composition so formed is fed to a screw-type plasticating extruder wherein the combination is mixed, homogenized, and formed into a sheet or film by melt extrusion substantially according to the methods taught in copending U.S. Pat. No. 6,287,722 which is herein incorporated by reference to the entirety.
In an alternative preferred embodiment an electrode film of the invention is formed from 65 parts graphite mesocarbon microbeads such as MCMB, 3.25 parts carbon black, and 10 parts polyvinylidene fluoride-hexafluoropropylene (PVDF/HFP) copolymer such as Kynar FLEX(copyright) 2801 (Elf Atochem) as polymer binder, and the remainder dibutyl phthalate (Aldrich) as a plasticizer for the binder polymer. One method for forming the preferred electrode film of the invention is to disperse or dissolve the components thereof in acetone, or other suitable solvents for PVDF/HFP, by heating up to ca. 60xc2x0 C. to form a mixture followed by applying the mixture as a coating on a suitable substrate such as Mylar(copyright) polyester film (DuPont Company). Any means for coating the substrate may be employed such as solution casting using the well-known doctor-blade technique. The thus coated substrate is dried preferably at temperatures up to ca. 60xc2x0 C. under vacuum, and then calendered or otherwise subject to contact pressure to compress the electrode coating to form a smooth surface. The dibutyl phthalate plasticizer is extracted by immersing the dried coated substrate into a volatile solvent such as diethyl ether or methanol for at least 15 minutes followed by drying under mild vacuum at room temperature for at least one hour. The film is separated from the substrate before or during the extraction step.
The thus dried and extracted film can then be immersed into an electrolyte solution preferably a 1.0 M solution of LiPF6 in a solvent comprising a diester of the present invention.
It is found in the practice of the invention that ether/esters such as are taught by Webber, op. cit., are less oxidatively stable than the diesters, so that they degrade after fewer charge/discharge cycles, and are therefore less preferred. An example of such ether/esters would be 2-ethoxy ethyl acetate.
The lithium-ion cell of the present invention comprises a positive electrode, a negative electrode, and a separator, at least one of which, preferably all of which, will be in ionically conductive contact with the electrolyte solvent of the invention. The lithium-ion cell will also contain current collectors typically composed of either foils or meshes or metallized plastics where the metal is composed of aluminum (for the cathode) and copper (for the anode). One of skill in the art will recognize that under normal operating circumstances, all of the components of the cell will be in said contact, since it is by virtue of said ionically conductive contact among the components of the cell that the cell operates.
The positive electrode of the lithium-ion cell of the present invention is preferably a mixture of the preferred diester of the invention and a lithium-containing transition metal oxide which is capable of absorbing and releasing lithium-ions to a capacity of  greater than 100 mAh/g such as LiCoO2, LiNiO2, LiNixCoyO2, and LiMn2O4.
The lithium-ion cell of the invention may be formed by any means such as is known in the art. The components of the cell may be first combined in the dry state, with the electrolyte solution added as a late step in the process. Or, the electrolyte solution may be added at any step in the process.
In a preferred method for forming the lithium-ion cell of the invention, as described in copending U.S. Pat. No. 6,287,722, which is incorporated herein by reference to the entirety, the electrolyte solvent of the invention is first mixed with an ionomer and such other ingredients as are necessary or preferred in the composition of the particular cell component being formed. The resulting composition is then subject to a film formation step by melt extrusion employing a screw-type extruder.
The other components of the lithium-ion cell of the invention may be formed in a similar fashion. The negative electrode is preferably formed by combining graphite powder, carbon black, the ionomer resin, and the electrolyte solvent of the invention and extruded into a film or sheet. Similarly, the separator is formed by extrusion of a mixture of the electrolyte solvent and the preferred ionomer, the mixture then extruded into a film or sheet.
In the most preferred embodiment, the several layers of the different components of the lithium-ion cell of the invention are laminated together in a continuous process.
It is known in the art that under some circumstances small quantities of additional solvents may provide improvements in battery properties such as high and low temperature behavior and cyclability. It may therefore be found desirable to combine the preferred mixture of dimethyl succinate and ethylene carbonate with an additional component chosen from the cyclic carbonates (other than EC), acyclic carbonates, or acyclic esters.